Cytology is a branch of biology involving the study of the formation, structure, and function of cells. As applied in a laboratory setting, cytologists, cytotechnologists, and other medical professionals make medical diagnoses of a patient's condition based on visual examination of a specimen of the patient's cells. A typical cytological technique is a “Pap smear” test, which involves scraping cells from a woman's cervix and analyzing the cells in order to detect the presence of abnormal cells, a precursor to the onset of cervical cancer. Cytological techniques are also used to detect abnormal cells and disease in other parts of the human body.
Cytological techniques are widely employed because collection of cell samples for analysis is generally less invasive than traditional surgical pathological procedures such as biopsies. Biopsies typically involve excising tissue specimens from the patient using specialized biopsy needles having spring loaded translatable stylets, fixed cannulae, and the like. With cytological techniques, on the other hand, cell samples may be obtained from the patient by a variety of techniques including, for example, by scraping or swabbing an area, or by using a needle to aspirate body fluids from the chest cavity, bladder, spinal canal, or other appropriate area. Cell samples are often placed in solution and subsequently collected and transferred to a glass slide for viewing under magnification. Fixative and staining solutions are typically applied to the cells on the glass slide, often called a cell smear, for facilitating examination and for preserving the specimen for archival purposes.
Machine vision devices, such as automated imaging and reviewing microscopes, have been utilized to acquire images of cell samples and to analyze the samples. Microscopes, including microscopes that are part of integrated imaging and review systems, require periodic calibration. Calibration procedures include positional calibration, which involves confirming that the actual position of a microscope stage and/or microscope slide is the position as indicated by the system, and optical calibration, which involves confirming that optical parameters are as indicated by the system.
In the past, calibration has been performed using an independent microscope slide that includes calibration components. During use, the calibration slide is placed into position on the microscope stage, calibration is performed, and the custom microscope slide is removed. The same calibration slide may be used to calibrate other microscopes.
Another known calibration system is described in U.S. Pat. No. 5,367,401 to Saulietis. Saulietis describes factory calibration of a microscope that involves mounting a stage having multiple slide slots on a motor assembly and using a motor assembly to place calibration targets in line with a viewing window. The position reading for each target and known distance data provide stage reference coordinates. Using a calibration slide in each slide slot, a target reference point on the calibration slide is aligned with a viewing window, and its coordinates are read to provide an absolute position relative to stage coordinates determined by the stage's target references during factory calibration. The absolute reference coordinate position for each calibration slide can then be stored with an associated serial number. With this technique, an end user is not required to perform any calibration of individual slide slots, but enters a serial number and relies on stored data.
Known calibration devices and techniques, however, can be improved. Various known devices, e.g., as described by Saulietis, still use specialized calibration slides and may only be capable of positional calibration (x, y, z, θ). Specialized calibration slides can be lost, damaged or destroyed. Individual calibration slides may also involve time consuming and inconvenient calibration procedures, which may be required at frequent calibration intervals. Additionally, calibration slides may suffer from degraded optical performance due to the environment, e.g., dust, dirt, grease, etc. Special calibration slides may also vary from one slide to another. For example, different calibration slides may have different thicknesses. These variations must be accounted for during calibration.